The invention relates to a magnetic recording and reproducing apparatus for magnetically recording and reproducing a video signal or the like on a magnetic tape, and more particularly to a tape feeding system therefor in which various kinds of cylindrical tape guide posts are so arranged as to pull the magnetic tape out of a cassette which contains therein supply and take-up reels mounted on spaced parallel axes to lead it along a path parallel with the datum plane parallel to the upper or lower surface of the cassette and wind the tape partially and spirally around a cylindrical tape guide drum (hereinafter simply referred to as a drum) which contains one or more rotary magnetic heads therein, and which is so arranged as to feed the tape smoothly and without jitter, flutter, ripple and other mechanical distortions.
A typical tape feeding system to feed the tape smoothly and stably from the cassette along the tape travelling path parallel to the datum plane is shown in FIG. 1, and has a combination of a drum 11 inclined with respect to the datum plane, a pair of cylindrical tape guide posts 12 and 13 positioned adjacent the drum, and a pair of cylindrical tape guide posts 14 and 15 perpendicular to the datum plane, the posts 14 and 15 being provided for adjustment of the arcuate distance of interengagement between the tape and the guide posts 12 and 13. In FIG. 1, the tape is indicated only by the longitudinal tape center line 16.
The guide posts 14 and 15 arranged perpendicularly to the datum plane cause the tape running parallel with the datum plane to change its direction, but do not function to change the height and the parallelism of the tape with respect to the datum plane. Therefore, in order to feed the tape stably at the tape entrance and exit of the cassette, it is necessary to satisfy at least the following two conditions:
(A) Condition of Height of the Tape: Heights of the tape center line with respect to the datum plane at the point 121 where the tape initially engages the inclined post 12 and the point 131 where it leaves the inclined post 13 should be equalized.
(B) Condition of parallelism of the Tape: The tape center line in its direction of movement should is to be parallel to the datum plane in the tape feeding path between the tape exit from the cassette and the point 121 and between the point 131 and the tape entrance to the cassette.
A method for determining the mechanical dimensions of the arrangement of the structural elements of the apparatus so as to satisfy the two conditions (a) and (b) is disclosed in Japanese laid-open application for patent, No. Showa 47-5474. In accordance with this application, because the drum 11 and the inclined posts 12 and 13 are inclined in such a way that the axes thereof are parallel to each other, the tape travels in a twisted path between the inclined post and perpendicular post, although it travels stably in the path between the drum and the inclined posts. In the tape path between these posts, the tape length along the longitudinal center line becomes different from the length along the upper and lower edges thereof, whereby there occurs a stress distribution in the tape in the width direction thereof, resulting in the mechanical distortion of the tape. If the tape length between the inclined and perpendicular posts is large, the stress added to the tape by the mechanical distortion becomes sufficiently uniform that feeding of the tape is not influenced by the stress. However, where these two posts are mounted in pairs on a loading arm which is provided for pulling the tape out of the cassette and wrapping the tape around the drum, it is necessary to arrange these posts close to each other because they have to return to the cassette opening in the tape unloading condition. Therefore, in such an arrangement, it is difficult to avoid causing excessive stress in the tape, thereby giving rise to above described difficulty.
Furthermore, when the apparatus is in a recording or reproducing mode, the tape pulled out of the cassette travels along the predetermined path through tape guiding means (not shown in Figure) such as a limiter post, an erase head, a tape tension control arm, etc., and is led to the drum including one or more heads therein and thence to a group of cylindrical guide posts. Thereafter, the tape is fed to the take-up reel in the cassette through a limiter post, an audio control head and tape driving means, composed of a capstan and a pinch roller, which drives the tape at a predetermined speed. More particularly, in order to obtain a high compatibility, the drum containing one or more rotary heads is formed with a step portion arranged spirally for engaging the edge of the tape and accurately fixing its position. For this reason, it is necessary to arrange limiter posts in the vicinity of the points where the tape initially engages and leaves the drum, for pressing the tape against the step portion so as to restrict the movement in the width direction thereof. In the event that each of these limiter posts is placed between the inclined post and the perpendicular post, if the tape is twisted therebetween, these limiter posts cause an abnormal stress distribution in the tape, thereby preventing stable tape feeding.
Thus, in order to feed the tape stably and without mechanical distortions, the tape feeding system must be so arranged as to satisfy not only the conditions (a) and (b), but also following condition.
(c) Condition of No twisting of the Tape: The tape is to travel without any twisting action in the path between the drum and the inclined post and between the inclined post and the perpendicular post.